The overall goals of the project are 1) to characterize B2-microglobulin (B2m) on the surface of lymphocytes, and specifically, 2) to characterize its relationship to other membrane components, especially those related to the histocompatibility system, 3) to define the B2m system in mice, and 4) to investigate the possibility that either the B2m-histocompatibility complex acts as a receptor for antigen on T cells or whether other molecules, structurally related to these histocompatibility antigens, in association with B2m may have this function. An additional goal is to determine whether quantitation of serum and urine B2m levels in clinical situations in which homograft rejection phenomena are taking place may be of diagnostic value as a sensitive assay for detecting the occurrence of graft rejections at a subclinical level due to immunological attack on membrane bound histocompatibility antigens. The goals set for the current year are 1) to define the B2m system in the mouse, 2) to evaluate the effect of antiserum to B2m on the mixed lymphocyte reaction and an antigen-induced proliferative response, and 3) to determine the value of quantitation of B2m levels in serum or urine as a sensitive assay in the follow-up of patients receiving organ transplants for detecting the early occurrence of graft rejection.